Injection Moulding, along with extrusion, ranks as one of the prime processes for producing plastic articles. It is a fast process and is used to produce large numbers of identical items from high precision engineering components to disposable consumer goods.  Materials such as polystyrene, nylon, polypropylene and polythene can be used in injection moulding. These are thermoplastics - this means when they are heated and then pressured in a mould they can be formed into different shapes.

Material Used: ABS, PA, PC, PP, GPPS

Most thermoplastics can be processed by Injection Moulding.

Application

Injection mouldings count for a significant proportion of all plastics products from micro parts to large components such as bumpers and wheelie bins.  Virtually all sectors of manufacturing use injection moulded parts - the flexibility in size and shape possible through use of this process have consistently extended the boundaries of design in plastics and enabled significant replacement of traditional materials thanks to light weighting and design freedom.  Products that can be injection moulded include: power tool housings, motor vehicle bumpers, disposable razors, telephone handsets, motor vehicle dashboards, washing up bowls, television cabinets, battery casings, wheelie bins, electrical switches, syringes, crates, DVDs, lids and closures.

The Process

The essential elements are as follows:

Material is introduced into the injection moulding machine via a Hopper.  The injection moulding machine consists of a heated barrel equipped with a reciprocating screw (driven by a hydraulic or electric motor), which feeds the molten polymer (material) into a temperature controlled split mould via a channel system of gates and runners.

The screw melts (plasticizes) the polymer, and also acts as a ram during the injection phase. The screw action also provides additional heating by virtue of the shearing action on the polymer.

The polymer is injected into a mould tool that defines the shape of the moulded part. 

The mould on this machine has been made to form plastic into a sphere.

1 . Granules of plastic powder (note the plastics listed above) are poured or fed into a hopper which stores it until it is needed. 2 . A heater heats up the tube and when it reaches a high temperature a screw thread starts turning. 3 . A motor turns a thread which pushes the granules along the heater section which melts them into a liquid. 4 . The liquid is forced into a mould where it cools into the shape (in this case a sphere). 5 . The mould then opens and the sphere is removed .

The pressure of injection is high: dependant on the material being processed it can be up to one thousand atmospheres.  Tools tend to be manufactured from steels, (which can be hardened and plated), and aluminium alloys for increased cutting and hand polishing speeds.  The costs associated with tool manufacture means that injection moulding tends to lend itself to high volume manufacture. 

The tool can be used to manufacture one consistent part in a repeating process or incorporate multi cavities (a multi impression tool), in which case many components can be manufactured on the same tool repeatedly with a single injection. 

Variants of the injection moulding process include:

• Multi-shot (or 2K moulding) where different materials are injected into the same mould eg: toothbrush
• Insert moulding where metal components are incorporated
• Structural foam moulding where the material is foamed to reduce density
• Assisted moulding where gas or water are incorporated to reduce wall thickness 

Computer Controlled
Modern day injection moulding machines are controlled by a built-in computer. Acting on sensor fed information, it controls all the actions of the machine and ensures consistent output and shot to shot quality.

Photographs of a Ferromatik Milacron - Maxima Moulding Machine and Mitsubishi Hydro-Mechanical Moulding Machine

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EXTRUSION

Extrusion is the production of an endless shaped plastics extrudate from powdery or granulated plastics moulding compounds.

In this process, an extrusion plant is used in which the extruder is the most important component. The operating principle of an extruder is that a screw rotates in a fixed, heated barrel. This screw is fed with plastics moulding compound through a radial opening in the cylinder and compresses, melts, homogenizes and forces the molten plastics mass from the exit or die opening.
The extruder is not in itself a plastics processing machine but is merely a very important link in the chain that makes up the extrusion line.

The extruder is fitted with a forming tool i.e. a die and this is followed by other required equipment such as for calibrating, haul-off, winding and cutting to form a complete extrusion plant.

In addition to the single screw extruders, there are extruders with multiple screws. Of the multi-screw extruder types the twin screw extruder is of great significance for the processing of powdery plastics, especially PVC .

Plastics Materials used in Extrusion :

In principle all thermoplastics can be extruded provided that they have a high viscosity in the molten state. This is necessary to hold the shape of the melt for a short while when it emerges from the die to prevent it from deforming. For this reason only certain thermoplastics are used for extrusion. Less viscous types have to be modified either through a high degree of polymerization or through the incorporation of specific additives. PVC followed by polyethylene and polypropylene are the most widely used thermoplastics in the extrusion process.

Available materials and typical use:

PVC (flexible PVC)

Hosing, gaskets, seals, fenders, pelmets, bumpers, handrails, sleeves, straps, trims, 'T' barbed edging strips

Cellular PVC      

Building and window products (i.e. cladding, skirting, architraves etc), wood replacement

ABS       

Water transfer tubing, cold rooms, freezer cabinets, toys

Polyethylene (high and low density)      

Toys, water and gas pipes, tubes, packaging, protective film

Polypropylene (co-polymer)      

Non load-bearing hinges, toys, tubing, protective plant and tree sleeves, oil pump tubes

Polycarbonate      

Light diffusers, low voltage light tubes for marking and directing applications, roof trims, lightboxes

PC/ABS alloys      

Electrical shrouds, profiles that call for superior flame retardance

Nylon 6      

Automotive applications, pump immersion tubes for barrels, pneumatic airline tubing for machinery.

Typical products manufactured using the extrusion process:

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Along with Injection moulding, Profile Extrusion of plastics materials is a widely used method of forming plastics materials. The Plastic raw material is both melted and traversed along by the action of heated rotary screws. It is a continuous process and is thus able to manufacture long lengths of a product. This is ideal for such applications as pipes and gaskets.  However, very often the continuous extrusion is cut into application lengths.

The Process

The extrusion process makes a continuous length of plastic the cross section of which is constant. This cross section is called the profile. A couple of examples to illustrate this are hosepipes and curtain tracks.
An electric motor coupled to a hydraulic drive continuously turns a screw, which is contained in the machines barrel. Plastic granules are fed into the hopper of the extruder and are drawn down into the screw. The barrel and screw are heated by external heating elements. As the plastic granules move along the screw they melt and are forced through a die which is located at the end of the barrel. The die contains the cross section of the profile of the extrusion required.

PVC profile window frames		Plastic extruded profile

Co-Extruded Profiles

This process combines two different materials or colours in a single profile. Special twin head extruders and purpose made dies are necessary. This technique is often used to replace two piece assemblies. The two materials are independently extruded and meet in the die, where being molten, they alloy at the point of interface.

Post Extrusion Working

To enhance a profile extrusion, post extrusion work can be carried out. Punching, Drilling, and Slotting are typical process. Bending & Forming and the application of hot roll on adhesive tapes are carried out off-line. Inkjet printing enables batch and product marking.

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Thermoplastic sheets are flat, plastic materials with a gauge of at least 250 microns and which include both flexible and rigid materials, as well as solid, foamed, and hollow materials.

Polystyrene continues to be the most common polymer for use in sheet extrusion. It is the dominant material for thermoformed packaging and competes with ABS and PP in technical markets. End use applications include tubs and pots for yogurt, margarine, and desserts. Thermoformed packaging is also used in many other applications in the food industry.

Within the building and construction industries, sheet extrusion is used for a variety of applications. One of the main uses of extruded PS sheet is for thermal insulation materials for walls, roofs, and under floors.

In the automotive industry, sheet is currently used to produce interior trim, panels, and dashboards. Foamed polyolefin sheet, both cross-linked and non-cross-linked, is also used in automotive applications.

There are a number of other applications where thermoformed sheet plays a significant role. These include the manufacturing of luggage, refrigerator liners, and shower units.

Sheet extrusion is typically used to create sheets with a thickness of 0.5 to 25 mm, and widths of two to nine meters. Polyethylene products fabricated using the sheet extrusion process includes:

• Landfill base and cap liners (2 – 2.5 mm thick)
• Tunnel liners for road or train tunnels (2, 3, 4 mm thick)
• Sheets for thermoforming
• Liners (0.25 – 2 mm thickness)
• Concrete protective liners and sheets for pipe relining

Applications:

Land Drainage Tubing’ Cable Harness; Light Diffusers ; Rainwater Pipes and Guttering Electrical Conduit and Cable Protector ; Curtain Tracks; Fridge Seals, Edge Trimming, Facias, Caravan Window surrounds, garden fence posts and decking; Animal Feeding Troughs; Door seals, Blood drip tubes and Catheter tubes, Gas-, water and soil pipes.

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The Cable extrusion sector covers a wide range of products, from insulation for optical fibres, household cables and high current cables, through to sheathing for heavy cables. In each case the conductor to be coated may be bare metal, or may already be coated with one or more layers of polymer.

The polymers used also cover a wide range, from plasticised PVC, polyethylenes and polyamides, througth to more exotic materials such as fluoropolymers.

Fibre and Monofilament Extrusion :

Synthetic filaments and fibres are usually produced via an extrusion process whereby the polymer (e. g., polypropylene, polyethylene, polyester, nylon, etc. ) is melted and forced through fine holes known as spinnerets. The fibres are then stretched or drawn until the required weight is achieved.

Typical polymers that may be processed are polypropylene (homopolymer and copolymer), polyethylene (low density, linear low density and high density) as well as blends thereof.

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One of the most common methods of film manufacture is Blown Film (also referred to as the Tubular Film) Extrusion. The process involves extrusion of a plastic through a circular die, followed by "bubble-like" expansion. The principal advantages of manufacturing film by this process include the ability to:

• produce tubing (both flat and gussetted) in a single operation
• regulate the film width and thickness by control of the volume of air in the bubble, the output of the extruder and the speed of the haul-off
• eliminate end effects such as edge bead trim and non uniform temperature that can result from flat die film extrusion

Blown Film Extrusion can be used for the manufacture of co-extruded, multi-layer films for high barrier applications such as food packaging.

Plastic melt is extruded through an annular slit die, usually vertically, to form a thin walled tube. Air is introduced via a hole in the centre of the die to blow up the tube like a balloon. Mounted on top of the die, a high-speed air ring blows onto the hot film to cool it. The tube of film then continues upwards, continually cooling, until it passes through nip rolls where the tube is flattened to create what is known as a ' lay-flat' tube of film. This lay-flat or collapsed tube is then taken back down the extrusion ' tower' via more rollers. On higher output lines, the air inside the bubble is also exchanged. This is known as IBS (Internal Bubble Cooling).

The lay-flat film is then either kept as such or the edges of the lay-flat are slit off to produce two flat film sheets and wound up onto reels. If kept as lay-flat, the tube of film is made into bags by sealing across the width of film and cutting or perforating to make each bag. This is done either in line with the blown film process or at a later stage.

Typically, the expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter. The drawdown between the melt wall thickness and the cooled film thickness occurs in both radial and longitudinal directions and is easily controlled by changing the volume of air inside the bubble and by altering the haul off speed. This gives blown film a better balance of properties than traditional cast or extruded film which is drawn down along the extrusion direction only.  

Materials used:

Polyethylenes (HDPE, LDPE and LLDPE) are the most common resins in use, but a wide variety of other materials can be used as blends with these resins or as single layers in a multi-layer film structure. these include pp, pa, evoh. In some cases, these materials do not gel together, so a multi-layer film would delaminate. To overcome this, small layers of special adhesive resins are used in between. These are known as “tie layers”.

Applications

Blown film can be used either in tube form (e.g. for plastic bags and sacks) or the tube can be slit to form a sheet.

Typical applications include Industry packaging (e.g. shrink film, stretch film, bag film or container liners), Consumer packaging (e.g. packaging film for frozen products, shrink film for transport packaging, food wrap film, packaging bags, or form, fill and seal packaging film), Laminating film (e.g. laminating of aluminium or paper used for packaging for example milk or coffee), Barrier film (e.g. film made of raw materials such as polyamides and EVOH acting as an aroma or oxygen barrier used for packaging food, e. g. cold meats and cheese), films for the packaging of medical products, Agricultural film (e.g. greenhouse film, crop forcing film, silage film, silage stretch film).

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Thermoforming has close similarities with Vacuum Forming except that greater use is made of air pressure and plug assisted forming of the softened sheet.

The process is invariably automated and faster cycle times are achieved than in the Vacuum Forming process. Only thermoplastics sheet can be processed by this method.

The largest application for thermoformed articles is for Food Packaging. Other industries include toiletries, pharmaceuticals and electronics. The modern food supply chain uses many forms of thermoformed articles, e.g.
meat trays, microwave & deep freeze containers, ice cream and margarine tubs, delicatessen tubs, snack tubs, bakery and patisserie packaging, sandwich packs and vending drink cups are just a few of the food related applications. Other non-food applications include manufacturing collation trays, blister packaging and point of sale display trays.

The Process

The majority of thermoforming production is by Roll Fed machines. Sheet Fed machines are used for the smaller volume applications. Larger production units have in house sheet extrusion equipment. Because of the complexities in synchronising sheet extrusion equipment and the thermoforming machines, the two processes can be carried out independently of each other. The extruded sheet being produced in advance of production schedules.

With very large volumes a fully integrated in-line, closed loop system can be justified. The line is fed with plastics raw material, with extruders feeding directly into the thermoforming machine.

The plastic sheet is softened at the heating station. It then indexes to the forming station where the mould tools are located. The forming of the sheet is by a combination of air pressure and male core plugs. Certain designs of thermoforming tool facilitate the cropping of the article being formed within the thermoforming tool. Greater accuracy of cut can be achieved by this method due to the article being produced, and the skeletal (scrap), not having to be re-positioned. Alternatives are where the formed sheet, including skeletal, are indexed to the cropping station.

The high volumes of articles being produced demand that a parts stacker is integrated into the forming machine. Once stacked the finished articles are now packed into boxes for transportation to the end customer. The separated skeletal is either wound onto a mandrill, for subsequent chopping, or passes through a chopping machine which is in line with the thermoforming machine.

Materials Used: Many thermoplastics can be thermoformed, they include Polystyrene, Polypropylene, PET, and PVC. EVOH is commonly incorporated into a co-extrusion for its superior barrier properties in food.  Co-extrusions of these materials are commonly used to provide precise properties for specific applications.

The demands of the food packaging industry are for materials which resist the passage of odours, moisture and gases, hence the use of plastics with superior barrier properties.

Multi-Layer … Mono Material

For non-demanding applications, a mono material sheet is usually specified. This is a sheet in which there is only one material. With a more demanding application, a multi-layer sheet is used.

Product Decoration

Many of the food related applications demand attractive point of sale product identification and decoration. This can be achieved by either the use of pre-printed sheet or post moulding printing. In the former, multicolour pre-printed sheet is located relative to the mould cavities before forming.

In the latter, the formed articles are printed on a separate printing machine. The limitations of product shape, restricts the extent and ability to print.

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Thermoforming is one of the oldest and most common methods of processing plastic materials. Vacuum formed products are all around us and play a major part in our daily lives.

The process involves heating a plastic sheet until soft and then draping it over a mould. A vacuum is applied sucking the sheet into the mould. The sheet is then ejected from the mould. In its advanced form, the vacuum forming process utilizes sophisticated pneumatic, hydraulic and heat controls thus enabling higher production speeds and more detailed vacuum formed applications.

Virtually all thermoplastics can be supplied as sheet and vacuum formed. The more commonly used materials are listed below.

Materials used: ABS, PETG, PS, PC, PP, PE, PVC, PMMA

Applications

Bath and shower trays, yoghurt containers, boat hulls, machinery guards, vehicle door liners, refrigerator liners, sandwich boxes, parts of vehicle cabs, exterior shop signage.

Nature of Use and Limitations

Vacuum forming offers several processing advantages over other forming processes. Low forming pressures are used thus enabling comparatively low cost tooling.

Since the process uses low pressures, the moulds can be made of inexpensive materials and mould fabrication time can be reasonably short. Prototype and low quantity requirements of large parts, as well as medium size runs therefore become economical.

More sophisticated machines and moulds are used for continuous automated production of high volume items like yoghurt pots, disposable cups and sandwich packs.

Unlike other thermoplastic forming processes, where powder or resin are the starting point, vacuum forming uses extruded plastic sheet. With vacuum forming a secondary process may be required to trim the formed sheet to arrive at the finished part. The trimmed waste can then be re-ground and recycled.

The Process

Below are the stages involved in the vacuum forming of a small 'plastic' dish or bowl

Films are cast by pouring a plastics solution from a slot die onto a rotating drum or an endless metal belt.

This process is now only used for the production of cellulose acetate film. For most applications, the films must be relatively soft and flexible. For this reason up to 5% plasticizer, mainly phthalic and phosphoric acid esters, is added to the pure cellulose acetate.

The solution is produced by means of a solvent mixture to achieve a low viscosity at a high solids content. Because of possible ignition and the formation of explosive mixtures with air, chlorinated hydrocarbons such as methylene chloride and chloroform are used in the solvent mixture for security reasons.

After dissolving, the mixture is pumped through filter presses to remove minute impurities. The casting machines are usually installed in gas-proof, closed housings to keep out dust particles emanating from the surrounding air. The casting equipment consists of a trough with a slot die. There are two types of casting machines available i.e. the non-pressure gravity casting machine and the pressure casting machine. Solvent is removed in a heating and vacuum station and is recycled for economic reasons. In most cases the solvent is removed with carefully prepared, heated drying air. The film exits through the machine housing via a slit and is wound up. This casting method is used for producing only relatively thin film. To make thicker cast films several layers are cast on top of one another.

Cast cellulose acetate films are used mainly where optical clarity is required. These films have a main application in the photographic and film industry. Within limits they are also used in the packaging industry and in electronics (condensers).

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Rotational moulding, also known as rotocasting or rotomoulding, is a low pressure, high temperature manufacturing method for producing hollow one-piece plastic parts. As with most manufacturing methods for plastics pasts, rotational moulding evolved from other technologies. The basic principle of forming a coating on the inside surface of a rotating mould dates back for many centuries, but the process did not gain recognition as a moulding methods for plastics until the 1940s.

A mould that can be heated is filled with the correct amount of moulding compound (usually powder or liquid) and as soon as the mould is heated, the compound starts melting onto the inner walls of the mould. During the heating and cooling process, the mould rotates round two axes, which are positioned at right angles to each other, to guarantee an even coating of the mould contours. The rotational speed can be up to 30 revolutions per minute. and is a low pressure process.

Hollow articles with complicated closed or open shapes with volumes of up to 15 000 L, diameters of 2 000 mm and lengths of 2 500 mm and more can be produced in the rotomoulding process. The advantages of this production method are:

• Low equipment and mould costs in comparison to blow moulding and especially injection moulding
• The ability to produce large articles that are almost free of stress and with even, adequate wall thicknesses

These properties are important for the production of for instance transport and storage containers for environmentally dangerous liquids such as chemicals and heating oil.

Rotationally moulded hollowware can furthermore be reinforced with materials such as glass fibres. For the moulding of such reinforced articles, the required amount of short chopped glass fibres is added and mixed with the moulding compound before charging into the mould. If the process is done correctly, then the chopped fibres will be evenly distributed in the walls.

Materials used for this process: LDPE, HDPE, PS, Polyacetal, Polyamide

Typical moulded parts include bulk containers, tanks, canoes, toys, medical equipment, automotive parts and ducts.

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Design and tooling constraints sometimes make it more economical and/or advantageous to tool the product as two or more pieces. Post moulding joining of the parts provides a means of achieving an end solution.

Plastic Welding is the joining of thermoplastics by means of heat and force with or without a welding additive (i.e. welding rods etc).

The Process

The plastics welding process has become important not only in the apparatus and pipe industries but also in the assembly of technical components. It is also of importance in the joining of building elements as well as in the welding of packaging films.

Welding takes place in the plasticised stage of the surface of the parts to be joined. The melt properties of the plastics material are therefore vital criteria for determining its weldability. Because of their large molecular mass and structure the following plastics cannot be plasticised at all or only with difficulty and are therefore technically not weldable: PTFE, cast-PMMA.

Because the surgaces of the parts to be joined have to be in a plasticised state a problem arises in that only those thermoplasts which can be plasticised under the same conditions can be welded together.

There are also rules which apply to specific materials. For the creation of a plasticised state a certain temperature, the welding temperature, is required and to ensure a close bonding of the joining surfaces a certain force or pressure, the welding pressure or force, is necessary. The welding temperature has to take effect over a certain period of time due to the poor heat conductivity of plastics i.e. the temperature effect time. This ensures that the heat reaches a sufficient depth in the parts to be joined. The term “temperature effect time” is replaced with the term “welding rate” in continuous welding operations.

The welding parameters of pressure, temperature and temperature effect time, or welding rate have to be adjusted to one another and must confirm with the material to be welded. This requires great skill on the part of the welder in all handwelding processes.

The joints have to be prepared before welding starts and the joining surfaces should be cleaned thoroughly. The individual parts then have to be fixed in such a manner that they cannot change their position relative to one another during welding. The welding process itself can be divided into the production steps of Plasticising, joining and solidification.

As with the plasticification and joining the solidification stage must also receive careful attention while maintaining exact welding parameters. Cooling must be done slowly and evenly otherwise the welded part may warp due to uneven stresses (residual welding stresses).

There are various methods of plastics welding such as ultrasonic-, butt-, socket-; hot air-; hot air extrusion-; electrofusion-; ultrasonic-; pvc solvent-; friction- and high frequency welding. For more information on these processes, we suggest that reference is made to “Plastics Processing”, by O C Vorster and W Rabe, 6 th edition, 1996, which is available to purchase from the Plastics Federation of SA.

Materials used: PE, PP, PVC, PC, PA, PMMA, ABS

Applications: pipes, tanks, sheets

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The efficiency of the extrusion blow moulding process in the manufacture of hollow containers is remarkable.

Under the term blow moulding, we understand the blowing-up of an extruded thermoplastic parison (a tube of molten material flowing from the die) in a two-piece hollow mould (negative mould) until the parison has accepted the shape of the cavity.

This is a two stage process: the first is the extrusion of a tube, and the second is the moulding of the tube in a blow station to form a hollow body.

A variation on conventional blow moulding is stretch blow moulding. In this process better product properties are achieved despite less material input. As a start only symmetrical hollow shapes made from PVC and thermoplastic polyester can be produced by this method.

This process differs from extrusion blow moulding in that a pre-form is stretched (Bi-axial orientation) in the thermo-plastic phase into its final shape during a second blowing stage. After cooling, a high-quality, thin-walled hollow article is obtained. Its characteristics have led to widespread use in the food packaging industry, for example, for cooking oil or mineral water.

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One of the main attributes of plastics materials is their ability to be moulded into a finished component with no need for subsequent work to be carried out. Complicated shapes, holes and undercut features can be moulded into the component using tooling and moulding techniques. However all this comes at a cost of tooling expense.

Why Machine Plastics?

Moulding tools and forming equipment used in the various plastic moulding processes are invariably hand made one off creations. They can often take weeks and months to manufacture with a resultantly high cost. Where a plastics component is specified and the numbers to be used are not large, then machining the component becomes more economical. Not all plastics materials can be machined. The more rigid a plastic then the easier it is to be machined. The more flexible and the softer plastics are not suitable for machining.

ADVANTAGES OF MACHINING PLASTICS

• No mould costs are needed
• Ability to manufacture plastic components with short lead times
• Ability to manufacture low volumes economically
• Can trial a design before committing to tooling
• Thicker wall sections can be accommodated
• Components too large to be moulded can be machined from fabricated plastic
• The forces required to machine plastics are low
• Plastics normally machine dry
• Swarf can be recycled back into the compounding process

DISADVANTAGES OF MACHINING PLASTICS MATERIALS

• Machining ability limited to the more rigid plastics materials
• Relative high cost of block plastic material
• High scrap (relative to other plastics forming processes) can result
• High volume of swarf to be removed can present difficulties
• High costs of CNC machine time
• Volume production by machining will require robust jigs and fixtures
• Plastics materials do not conduct away any heat generated in the machining process
• Dust producing composite plastics require an effective dust collection system
•  

Considerations When Machining Plastics Materials

Due to the softer nature of plastics materials, the holding jigs and fixtures have to be designed with jaws which protect the plastic being machined, this can be with other plastics materials shaped to the form of the block being machined. In addition the jigs require to be robust in order to support the material being cut.

Thermoplastic Plastics Materials being machined can be cooled with an air blast providing the resultant swarf is continuous and not in chipping form. Thermosetting plastics can be cooled using a liquid coolant, but care needs to be taken in terms of plastics prone to swelling in water to ensure that machined dimensions do not change. 

Heat generated in the process can cause thermal expansion – this effect must be factored in as dimensions may alter on cooling. 

Methods of Machining Plastics Materials:
CNC Machining

If the component to be cut has a complex shape, its profile can be programmed into a computer. A CNC machining centre can be used to manufacture duplicate numbers of components. Multiple interchangeable cutters typically used on CNC machines enable complex and varied components to be machined.

Turning
If the shape to be achieved is round, then a simple turning operation can be used. Specialist supplementary equipment attached to the lathe can extend the capabilities of the lathe's operation.

Milling
This method of machining can vary from simple milling to profile and CNC milling.
Again as with lathe work, either additions to the milling machine, or the use of a more complex milling machine can extend the milling machine's capability to make more complex shapes.

Sawing
Invariably this method of machining is solely for parting off sections of plastic material from bar stock for subsequent working by other machining operations.

Die Cutting

In certain cases the use of die cutting of plastics material can produce a simple component. The process is limited to sheet material. A male and female die are used to punch out a predetermined shape. The process can be either a manual process or automated using a special machine.

Hot Knife Cutting

The softer less rigid types of plastic can be cut using a hot knife to slice through the plastic. An electrically heated wire or blade melts the plastic locally. This type of process is commonly used to cut blocks of foam and Expanded Polystyrene (eps).

Punching
Certain shapes can be cut on metal type punching presses. Like a CNC machine, they are invariably computer controlled and are multi tool bit equipped. This process is limited to the thinner thermoplastic and thermoset sheet.

Water Jet Cutting

This process is used to edge trim fibre reinforced thermosetting components, which would otherwise prove difficult to trim by other processes. The tough reinforcing layers in the material defy trimming by conventional knives and cutting equipment. The narrow cutting path and fast progess without dust or chippings are an advantage.

Separating
Acrylic and laminated sheet can be separated by means of scoring using a sharp knife and breaking about the scored line.

Laser Cutting

This process can be used for cutting and profile boring of certain types of acrylic and other plastics although not thermosetting. The process uses an industrial laser to melt the plastic often with computer controlled profile following.

Ultra-sonic Cutting

Some of the softer thinner plastics can be cut using ultra-sonic equipment. The high frequency generated by ultrasonics in the tool, have the effect of locally melting the plastic being cut. Again, integrated with computer profile control the process lends itself to high speed automated production lines.

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Cooling is a critical element of the process and one of the areas that can be targeted for Energy Savings - for further information on cooling:

The necessity to cool or chill plastics processing machinery is mainly related to thermoplastic materials. At room temperature thermoplastic materials (polypropylene, nylon and PET etc) are solid. In order to shape them they must first be heated to their molten temperature. When molten, they can then be manipulated (injection moulded, extruded etc) to a new shape. When formed to their new shape they must then be cooled to solidify them. Considerable amounts of heat energy have to be extracted from the material, the tooling and the machinery that is doing the forming.

How is This Achieved?
Dependant on the process speed and the quantity of plastic material being moulded or extruded, process cooling can be applied in many ways.

Larger Plastics Processing Plants
The larger plastics processing plants generally have a centralized process water cooling or chilling system.  Pipes circulate the cooling water through the plastics machinery and subsequently the warmer return water back to the central cooling unit. These centralized chilling and cooling systems can either be refrigerant process water chillers or the forced air type, more commonly known as air blast coolers, depending upon the temperature of water required.

Smaller Processing Plant

Alternatively, independent refrigerant type water chillers can be sited next to a plastics processing machine to provide individual machine cooling.

Water Chillers /Air Blast Coolers
Generally the larger installations rely on the refrigerant based centralised chilled water systems. A combination of water chillers and air blast coolers can be installed to achieve low energy “free cooling” during low ambient conditions.

Refrigerated Water Chillers
The refrigerant gas in vapour form is compressed then cooled via the condenser to return it to a liquid state. It is then injected into one circuit of an evaporating vessel via a thermostatic expansion valve. Warm water from the process is circulating in the other circuit of the evaporator and the heat from the water transfers from one circuit to the other to evaporate the liquid refrigerant. The thermostatic expansion valve controls the flow of refrigerant into the evaporator according to the prevailing process heat load. Now as a vapour, the refrigerant gas returns to the compressor and the cycle continues. In this way water temperatures well below existing ambient air temperatures can be achieved, typically 5ºC to 15ºC.

Forced Air or Air Blast Water Coolers
These function very much like a car radiator. The warm process water is pumped through a coil matrix. Air at ambient temperature is forced over the fins of the coil effecting a transfer of heat into the air. The now cooled water is pumped back through the water cooling system to the plastics process machinery. This method of process cooling is entirely dependant on the ambient air temperature for its cooling efficiency and the water temperature achieved would normally be within 3º to 5ºC of the prevailing air temperature. Lower water temperatures can be achieved by using an “adiabatic air blast cooler” which in principle is the same but with the addition of a water spray system which at high ambient temperatures coats the fins with a water mist.

Cooling Plastics Processing Machines
Most plastics processing machines use hydraulic systems to control the machines moving parts. The hydraulic pump motor becomes hot when operating under load and heat from the motor will transfer to the oil via the motor shaft and pump casing. Further heating of the oil will result from the friction caused by the flow of the hydraulic oil through the valves, pipes and cylinders. All hydraulic oils have advisory maximum operating temperatures, so hydraulic oil cooling is one of the functions of plastics process cooling systems to ensure that the hydraulic oil is maintained at its optimum working temperature.

Cooling Plastics Products

The extrusion process has a water bath or spray system in which the hot plastic being extruded is immediately immersed. The bath or spray is connected to a chilled water system which creates the necessary heat transfer to solidify the plastic.

In the plastic blown film process, the air blown into the bubble can be chilled by passing it across a finned heat exchanger, which is supplied with chilled water. The air cooling or chilling system can have the effect of improving the clarity and quality of the film and speed at which it can be produced.

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Useful links:

• www.lenntech.com/polypropylene.htm
• www.knovel.com
• www.technologystudent.com